This invention relates to a probe for a magnetic resonance inspection apparatus.
A magnetic resonance inspection apparatus constituted by utilizing magnetic resonance imaging irradiates a radio frequency magnetic field so as to generate nuclear magnetic resonance at an arbitrary position inside a subject, and includes a probe for receiving the magnetic resonance signal generated inside the living body. In magnetic resonance imaging, efficiency of the probe and uniformity of the radio frequency magnetic field must be improved to increase measured image quality and to shorten the imaging time. A quadrature detection probe is known as one of the means for improving performance of the probe, as disclosed in "Journal of Magnetic Resonance", Vol. 69 (187), pp. 236-242. This probe will be referred to as the "QD probe" in this specification. The probe of this kind is also described in JP-A-61-230052.
FIG. 1 is a perspective view of this QD probe and FIG. 2 shows its wiring structure. As is obvious from these drawings, this QD probe comprises four I-shaped first conductors 100, 200, 300, 400, ring-like guard rings 11, 12, capacitors or capacitance elements 31-38, 41, 42, and power supply circuits 500, 600. The first conductors 100, 300 and the guard rings 11, 12 constitute a coil, that is, an induction element, and this coil has a parallel relation with predetermined capacitance elements. This induction element and the predetermined capacitance elements constitute one probe. On the other hand, the first conductors 200, 400 and the guard rings 11, 12 form another coil. This coil detects a signal whose phase deviates by 90.degree. from the signal detected by the former coil. This coil, too, has a parallel relation with the predetermined capacitance elements and constitutes another probe.
When a subject such as a human head is inspected, a radio frequency magnetic field is applied generally to induce a maximum current in the probe and improve the signal-to-noise ratio (S/N). Thus, EQU .omega.=.gamma.Ho
where .omega. is an angular velocity of the Larmor's precession, .gamma. is a gyromagnetic ratio and Ho is the intensity of a static magnetic field.
The intensity Ho is determined on the basis of this relation so that .omega. becomes the resonance frequency of the probe. For example, Ho is set to 21.1 MHz.
The studies conducted by the inventors of the present invention reveal that the resonance characteristics of the probe described above are such as shown in FIG. 3. In the diagram, an impedance .vertline.Z.vertline. can be determined from the reactance component due to the coil and the capacitance elements and from the resistance component of a subject and the conductor itself in one probe. A first peak 74 represents resonance of the probe. A second peak 73 appears immediately adjacent to the first peak. Although the reasons why this second peak appears have not yet been clarified, it is assumed that another probe, that is, the probe whose phase deviates by 90.degree. from that of the probe which measures the impedance, affects the impedance due to the structure of the QD probe shown in FIG. 1. The first peak 74 is affected by the second peak 73 and the waveform of the first peak 74 becomes asymmetric with 21.1 MHz being the center.
When the first peak 74 undergoes deformation, the magnitude of the induced current which is distributed at frequencies near 21.1 MHz and detected on the basis of a gradient field produces a detection signal which is distorted due to deformation of the impedance.